The present invention relates to the field of magnetic resonance imaging (MRI) systems and, more particularly, concerns radio frequency (RF) coils for use in such systems.
In MRI systems or nuclear magnetic resonance (NMR) systems, radio frequency signals are provided in the form of circularly polarized or rotating magnetic fields having an axis of rotation aligned with a main magnetic field. An RF field is then applied in the region being examined in a direction orthogonal to the static field direction, to excite magnetic resonance in the region, and resulting RF signals are detected and processed. Receiving coils intercept the radio frequency magnetic field generated by the subject under investigation in the presence of the main magnetic field in order to provide an image of the subject. Typically, such RF coils are either surface-type coils or volume-type coils, depending upon the particular application. Normally, separate RF coils are used for excitation and detection, but the same coil or array of coils may be used for both purposes.
Conventional MRI systems have a number of artifact problems. For example, aliasing of unwanted signals into the resonance object image is a common problem in MRI applications. A particular form of artifact, sometimes referred to as an aliasing artifact, can occur in the either the frequency direction or the phase direction within MRI systems. In this type of artifact, an area of anatomy that is at least partially within the excitation field of the body coil has a local Larmor frequency identical to a pixel within the imaging field of view. This phenomenon typically originates from areas outside the field of view, but causes artifacts inside the image. It often arises as a result of the non-linearity of the gradient fields and/or non-homogeneity of the DC magnetic fields.
Accordingly, to reduce the occurrences of unwanted artifacts, there exists a need for MRI power can be lost in cables and switches before the power gets to the transmit coil.
Another advantage of the present invention is that different RF field shapes can be generated with a single coil thereby enabling selective excitation within the imaging volume.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.